Nixpkgs: APL keyboard layout not accessible

[Fresheyeball]

Describe the bug
The apl keyboard does not work

To Reproduce

services.xserver.layout = "us,apl";
services.xserver.xkbOptions = "grp:swtich";

you can also see this by running

$ setxkbmap apl
Error loading new keyboard description

Expected behavior
It's possible to switch to the apl keyboard layout.
In this case by holding the right alt key.

Additional context
This does work for other layouts such as us,dvorak, and worked before 19.08

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.14, NixOS, 19.09.1957.57c35c63004 (Loris)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.4`
 - channels(root): `"musnix, nixos-19.09.1957.57c35c63004"`
 - channels(isaac): `"nixos-18.03-18.03.133402.cb0e20d6db9, nixos-19.03-19.03.173684.c8db7a8a16e"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

[Fresheyeball]

Learned something new:

services.xserver.layout = "us,apl";
services.xserver.xkbOptions = "grp:swtich";
services.xserver.xkbVariant = ". dyalog";

won't build at all, with:

building '/nix/store/8invs2q7lcd7ikl9qdj8db9m555ipibv-linux-5.4.24-modules.drv'...
The XKB keyboard definition failed to compile:
    apl:484:13: syntax error
    Couldn't process include statement for 'apl(dyalog)'
    Abandoning symbols file "(unnamed)"
    Failed to compile xkb_symbols
    Failed to compile keymap
/nix/store/pw4nx7rsjrpllss2zzcgm6763jf1ny65-ckbcomp-1.193/bin/ckbcomp: instead of "hidden"dyalog_codes"{key<BKSP>{[Uf850,Uf855]};key<" in apl expected string.
copying 29 paths...
copying path '/nix/store/difa7ky29gwr8ygr39iqnipns2gnacx5-nixos-icons-2017-03-16' to 'ssh://goldwater'...
builder for '/nix/store/4i53vhn66zpkdr3kr314yd3ffj8w1a3d-xkb-console-keymap.drv' failed with exit code 255
builder for '/nix/store/aid6p7mm5kn3fsb26php5bi6vkfiyd4j-xkb-validated.drv' failed with exit code 1

[caadar]

Can't load apl layout too:

$ setxkbmap apl
Error loading new keyboard description

My setup:

- system: `"x86_64-linux"`
 - host os: `Linux 5.4.30, NixOS, 20.03.git.0bb35152be8 (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.2`
 - channels(user): `""`
 - channels(root): `""`
 - nixpkgs: `/src/nixos/nixpkgs`

(Found this bug report when try to build custom layout with services.xserver.extraLayouts, without success though...)

[caadar]

Another standard layouts from .../xkb/symbols/ directory seems in the good state.

[paulvictor]

I am also facing the same issue. A couple of findings, setting the variant as common allows xkbvalidate to accept the config. Another observation is running a tail -f /var/log/X.0.log shows a line

Error loading keymap /tmp/server-0.xkm

every time setxkbmap apl is run. Any pointers from here?

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